Issue #150, August / September 2012

Intermediate

- Intermediate

“Charges” (electrons in wires) are the energy carriers in an electrical circuit. Charges are part of the material of the conductor, the atomic structure of the copper or aluminum wire. Charges don’t leave the circuit, and they aren’t used up. They move from one atom to the next, with another charge filling the “hole” just vacated.

In a direct current (DC) circuit, they move in one direction. In an alternating current (AC) circuit, they move back and forth. In AC circuits, the direction of charge flow reverses many times a second. The voltage (electrical pressure) and amperage (rate of charge flow) go from zero to maximum in one direction (“positive”), back to zero, to the maximum in the other direction (“negative”), and then back to zero. We call this a “cycle,” and in the United States, AC is 60 cycles per second (Hertz or Hz for short). When the voltage and amperage peak and then go to zero at the same time, we say that they are “in phase” (see diagram). This is what happens in circuits that have only resistance.

But many AC circuits also have a couple of other electrical properties—inductance (storing energy in an electromagnetic field, opposing a change in amperage) and capacitance (storing energy in an electrostatic field, opposing a change in voltage). These push the voltage and amperage out of phase with each other, so that they peak at different times. In these “reactive” circuits, some of the energy is bounced back at the source in a delayed reaction, due to the characteristics of inductance and capacitance. Reactive loads include motors, fluorescent light ballasts, and many electronic devices.

Power, the rate of energy flow, can be calculated by multiplying voltage and amperage (electrical pressure and charge flow rate). So in a purely resistive circuit, if source voltage is 120 and amperage is 10, the power is 1,200 watts.

In a reactive circuit, because the voltage and amperage are not in phase, less power is available to the load. Some of the charges are just moving energy back and forth unnecessarily, and this creates an illusion of power, known as “reactive power.”

We call the product of volts times amps in a reactive circuit “apparent power” (also called “volt-amps”, and abbreviated “VA”). We call the power that is usable to the load “true power” (watts, abbreviated “W”). The ratio of true power to apparent power is called “power factor” (PF). A power factor of 1—when the apparent and true power are the same—is ideal.

W ÷ VA = PF

If we take the same 120 volts and 10 amps from the example above, but the load in the circuit has a PF of 0.8, the power available to the load will be only 960 watts. To make up the difference, higher amperage is needed in the circuit. Since losses in a circuit are tied directly to the charge flow rate (amperage), raising the amperage (to compensate for low PF) in a given size of wire means that the voltage losses in the wire will increase. So a circuit with bad (low) PF will need larger wires to keep the voltage losses at the same level as a circuit with good (high) PF.

Devices that purport to save you lots of money by correcting power factor are largely a scam. While avoiding poor PF or correcting it is not a bad idea, the reality is that utilities do not usually charge residential consumers for VA-hours, but for watt-hours. These devices (if they do correct for PF) may help the utility, but won’t help you much in most cases.

“Watts” are a measure of the energy flow from the generating source to the load. “Volt-amps” are a measure of the theoretical maximum energy flow, including the illusory reactive energy that is bounced back to the generating source. The practical lessons are that high PF devices are always going to be easier on your generating sources—they will increase efficiency in your systems. And wire sizing must take into account the full apparent power that the load needs, even though some of it is recycled by the circuit.

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